This invention relates to a driving force distribution control technique intended to vary the distribution of driving force to be transmitted from an engine to front and rear wheels in a four wheel drive vehicle. Particularly, it relates to a hydraulic pressure supply device of the motor drive type which is suited to be made compact, and to a fail-safe technique for coping with a failure of the device.
A driving force distribution control technique of this type is intended for improving the safety of driving a vehicle. For example, it is a technique for varying the distribution ratio of a driving force transmitted from the engine such that the driving mode of the vehicle is changed from a completely rear-wheel drive mode to a front-wheel and rear-wheel drive mode at the ratio of 50:50.
In such driving force distribution control technique, there is a provision of a variable torque clutch device, such as a multiple disc clutch, located on that portion of a driving force transmission path leading from the engine to front or rear wheels, so that a magnitude of the driving force to be transmitted (namely, driving force distribution) is varied by controlling a gain of the engagement force of the variable torque clutch device. The means for controlling a gain of a clutch engagement force includes an operating cylinder serving as an actuator, and a hydraulic pressure supply device (or unit) for supplying a hydraulic pressure to the operating cylinder in accordance with a command from outside. Conventionally, a hydraulic pressure supply device generally comprises a pump serving as a hydraulic pressure source, an accumulator which stores, at a predetermined pressure, a fluid which has been discharged from the pump, and a proportional solenoid valve for regulating the hydraulic pressure in the accumulator in accordance with a command from a controller and outputs it to an operating cylinder side. The conventional techniques so far discussed are disclosed, for example, in Japanese Patent Publication No. Hei 4-24253, Japanese Patent Unexamined Publication No. Hei 4-397335, U.S. Pat. No. 4,773,500, or U.S. Pat. No. 4,986,388.
However, those conventional hydraulic pressure control devices have several difficulties in making the control device compact and simplifying the controlling procedure with the use of it. The most serious difficulty among them is as follows. Since a pump and an accumulator are essentially employed, the hydraulic control unit itself becomes large in size and this makes it difficult to fulfill the requirement to make the overall system compact.